This invention relates to a control apparatus for internal combustion engines and, more particularly, to apparatus for controlling the fuel air ratio and the ignition timing of a lean-burn engine so as to keep them within the target range.
In a control apparatus for internal combustion engine, for example, a conventional lean burn control apparatus detects the air fuel ratio controlled by a lean sensor which produces a signal corresponding to the concentration of the oxygen in exhaust gas and controls the detected air fuel ratio so as to be the same as the target lean air fuel ratio, as described in Japanese Patent Laid-Open No. 279747/1987.
The target lean air fuel ratio is set at a specific air fuel ratio within the air fuel ratio range defined by the misfire boundary air fuel ratio of an engine and the air fuel ratio determined by the NOx limit necessary to clear a regulation of exhaust gas, namely, at a specific air fuel ratio within the target control range.
In the above-described prior art, however, a change of an engine with time is not taken into adequate consideration, and the target lean air fuel ratio set in the above-described way keeps a constant value irrespective of a change of the engine with time. Since it is expected that a change of the engine with time, a change in fuel property, a change in the atmosphere condition and the like move the misfire boundary to the high air fuel ratio side, the target lean air fuel ratio is conventionally set in advance at a value on a fuel rich side of the target control range (in FIG. 3) in consideration of the above-described movement of the misfire boundary. This fact makes a sacrifice of fuel cost and brings about a problem such as the increase in an amount of exhausted NOx with the increase in the weight of the car body, thereby making it difficult to clear the regulation of exhaust gas.